Recently, the demand for enhancing fuel economy of automobiles has prominently increased. As the lubricating oil compositions to be used for lubricating engines mounted in gasoline engine-mounted automobiles and diesel engine-mounted automobiles, engine oils of a low viscosity type giving good fuel economy have been required. At present, a lubricating oil composition of SAE viscosity grade of 0W20 showing a high temperature-high shear viscosity (HTHS viscosity determined at 150° C. under the condition of a shear rate of 106/s) of approx. 2.6 mPa·s is employed in practice.
It is noted, however, that the engine oils for four cycle gasoline engines of motorcycles are also employed for lubricating transmission system. Therefore, increase of wear (namely, lowering of anti-wear performance) of transmission gears and the like of the transmission system may be problematic. In consideration of these possible troubles, JASO T903-2006 indicates that the engine oils for four cycle gasoline engines of motor cycles should have a HTHS viscosity (at 106/s) of 2.9 mPa·s or higher. It has been noted, however, that the known engine oils of SAE viscosity grade 0W20 cannot give such high temperature-high shear viscosity. For this reason, the conventional engine oils of low viscosity type for four cycle gasoline engines of motorcycles only satisfy SAE viscosity grade of 10W30, 5W30 or 0W30.
As for engine oils for diesel engines mounted on motor cars equipped with a diesel particulate filter, ACEA C1-08 & C2-08, that is, European Specifications for the engine oils, indicate that the lower limit of the high temperature high shear viscosity (at 106/s) is 2.9 mPa·s s and that the upper limit of Noack evaporation loss is 13%.
Patent Publication JP 6-306384A describes a fuel economy type-lubricating oil for internal combustion engines comprising a mineral base oil showing a kinematic viscosity of 3-5 cST (at 100° C.), a viscosity index of 135 or higher, and a paraffin/total hydrocarbon ratio (namely, % Cp) of 90% or more and a specific amount of an organic molybdenum compound.
Patent Publication JP 2003-505533A describes a low-volatile fuel economy type lubricating oil composition showing NOACK volatility of 15 wt. % or less, which comprises at least 50 wt. % of a mineral oil, at least 95 wt. % of a saturated product and not more than 25 wt. % of naphthenes, which shows a kinematic viscosity of 4.0-5.5 mm2/s, a viscosity index of at least 120 and NOACK volatility of 15.5 wt. % or less, a specific amount of a calcium-containing detergent and a specific amount of an oilsoluble organic friction modifier.
Patent Publication JP 2000-87070A describes an engine oil composition for four cycle engine motorcycles showing low oil consumption and good fuel economy, which comprises a hydrocarbon base oil showing a kinematic viscosity of 3-10 mm2/s (at 100° C.) and a viscosity index of 120 or higher or a mixed base oil containing 15 wt. % of more of the hydrocarbon base oil, a zinc dialkyldithiophosphate, a metal-containing detergent, an ashless dispersant, a friction modifier and a viscosity index improver (which imparts a kinematic viscosity of 9.3-16.5 mm2/s at 100° C. to the oil composition). Patent Publication JP 2000-87070A further describes engine oil compositions of SAE viscosity grade of 10W30 and 10W40 for four cycle engine motorcycles.